The present invention relates to pre-expanded particles of a polypropylene resin suitable for the production of thermal insulation materials, cushioning packaging materials, returnable delivery boxes, automobile bumper cores and so on, a process for producing the pre-expanded particles, and a process for producing in-mold foamed articles from the pre-expanded particles.
In-mold foaming products obtained from polypropylene resin pre-expanded particles are superior in chemical resistance, heat resistance and distortion restoration rate after compression as compared to in-mold foaming products of polystyrene resin pre-expanded particles, and are also superior in dimensional accuracy, heat resistance and compression strength as compared to in-mold foaming products of polyethylene resin pre-expanded particles. For the reasons, polypropylene resin in-mold foaming products are widely used as automobile bumper cores, various packaging materials and others.
A propylene-.alpha.-olefin random copolymer (typically propylene-ethylene random copolymer) having a melt flow index (melt index: hereinafter referred to as "MI") of 0.1 to 25 g/10 minutes and an .alpha.-olefin content of not less than 1% by weight has been popularly used as a base resin of polypropylene resin pre-expanded particles for preparing foamed articles as mentioned above due to restriction in foaming properties and in-mold foaming by steam (for example, as disclosed in Japanese Patent Publication Kokoku No. 59-43492 and No. 2-50495). However, strictly speaking, in-mold foamed articles made from polypropylene resin pre-expanded particles are not so good in dimensional accuracy. They have a problem that the shrinkage in the direction of thickness is larger than that in the longitudinal direction depending on the shape of molded articles, so the so-called sink that the central portion of molded articles becomes thin generates to impair the appearance.
Also, in recent years, integration of grades of polypropylene resins has been actively made between manufacturers of polypropylene resins. Under these circumstances, it is difficult to freely select characteristics of polypropylene resins, such as MI, melting point and comonomer content, which are important for carrying out the in-mold foaming of polypropylene resin pre-expanded particles. In case of using such general purpose grade polypropylene resins in the in-mold foaming of pre-expanded particles, there arise problems that sink generates in the products and the molding time of in-mold foaming becomes long.
It is an object of the present invention to provide polypropylene resin pre-expanded particles which can be molded in a mold in a short molding time with good melt adhesion of the particles and without generation of sink to give cellular molded articles having good surface property and good dimensional accuracy.
A further object of the present invention is to provide a process for producing polypropylene resin pre-expanded particles which can be molded in a mold in a short molding time with good melt adhesion of the particles and without generation of sink to give cellular molded articles having good surface property and good dimensional accuracy.
Another object of the present invention is to provide a process for producing polypropylene resin in-mold foamed articles having good surface property and good dimensional accuracy in a short molding time with good melt adhesion of the particles and without generation of sink.
These and other objects of the present invention will become apparent from the description hereinafter.